1. Field of the Invention
The present invention relates to a butterfly valve arrangement used with a fluid pipe, and seeks to provide a valve housing which is obtained by the plastic working of a metal tube used as the raw material. This valve housing is reduced in weight and improved in accuracy.
2. Prior Art
So far, butterfly valve housings have been made by machining metal castings. Problems with these cast valve housings that they are increased in weight, have defects such as cavities, are expensive to process, and so on. Recently, such butterfly valve housings have been mass-produced by press-working metal sheets or tubes to obtain a plurality of parts, welding them together and machining the resulting integral pieces to impart the required accuracy thereto, as typically set forth in Japanese Patent Laid-Open No. 62(1987)-63279 (hereinafter referred to as the prior invention).
This prior invention may solve the problems associated with the conventional cast valve housings to some extent, but poses a grave problem in terms of the structure of the valve housing bodies and how they are made. This will now be explained with reference to FIG. 4, which is a sectional view of a butterfly valve used with the valve housing according to the prior invention. An inner shell 44 is formed by a press-working process which causes stress to remain in the largest amount among various working processes, and both ends of the inner shell 44 where residual stress is maximized and most likely to be released are arc-welded to both ends of an outer shell 45. Thus, thermal and residual stresses interact, giving rise to serious strain. It is this strain that has an adverse influence on the degrees of concentricity of bearings and the roundness of the inner circumference of the valve housing, intruding on smooth rotation of the valve or complete sealing of a fluid. Also, welding of upper and lower bearing bushes 48 and 47 to the inner shell 44 results in a further increase in thermal strain. The above welded portions are all functionally important and so should be additionally processed as by machining to impart accuracy to them, polishing to rid their surfaces of oxides and heat treatments to adjust their metal structure. Nonetheless, the valve housing according to the prior invention is far from that obtained by casting and post-machining in terms of accuracy. In order to provide valve housings having the high accuracy required for automatic regulating or high-pressure valves, which will be in great demand from now on, much more elaboration is still needed.